Absorbed dose distributions in lineal energy for neutrons and gamma rays were measured by using both a tissue-equivalent walled counter (TEPC) and a graphite-walled low pressure proportional counter (GPC) in the Am-Be neutron source facility at UOIT. A series of measurements were performed with the counters filled with propane-based TE gas (55.1% C3H8, 39.5% CO2 and 5.4% N2) at operating gas pressures corresponding to tissue spheres 2.0 , 4.0 and 8.0 μm in diameter. The results of these measurements indicated satisfactory performance of counters to measure microdosimetric spectra extending down to event-sizes that cover the gamma component of a mixed field. The spectra and the related mean values ̅y F and ̅y D are compared with other similar work but with monoenergetic neutrons of different energy range, the agreement between them is good. An assessment of the performance of different size TEPC has been done. An excellent agreement between their event size spectra was found and the proton edge appears at the same position on the lineal energy scale and differences in microdosimetric parameters ̅ and ̅ is not exceeding 3%, which is in the region of counting statistics. In Am-Be neutron field, the efficiency of the TEPCs was measured to have an average value of 250 counts per μSv or equivalently about 4.17 counts per minutes per μSv/hr. This efficiency is reasonable for dose equivalent measurements but needs a long integration period. The measurements showed that the dose equivalent which depends on the measurement of energy deposition by the secondary charged particles was originated mainly from elastic collisions of the incident neutrons with hydrogen atoms. Moreover the number of events in the sensitive gas is dominated by proton recoils. A non- negligible fraction of the dose equivalent resulted from gamma interactions, alpha and recoil nuclei. The energy deposition patterns in these micro-scale targets are strongly dependent on radiation quality, so differences of linear energy transfer (LET) of the components in a mixed radiation field are significant. Accordingly, in a radiation field with an unknown gamma ray energy spectrum, absorbed dose for neutrons can be obtained by the separation of neutron induced events from gamma events using their distribution in lineal energy. To separate neutron dose from gamma dose a simple lineal energy threshold technique has been used in addition to a more sophisticated methods using γ-fitting and the graphite-walled counter measurements. The results of this study will establish the degree of error introduced by using a lineal energy threshold, which is likely to be used in any hand-held neutron monitor based on TEPCs. / UOIT

A dosimetria de campos mistos de nêutrons e gamas é uma área de pesquisa que apresenta grande oportunidade de estudos devido ao aumento da utilização de procedimentos médicos como protonterapia e Terapia de Captura de Nêutrons (NCT Neutron Capture Therapy), além da importância para cálculo de doses ocupacionais e dos campos de irradiação em reatores nucleares. Para a dosimetria de campos mistos de nêutrons e gamas, a Comissão Internacional de Unidades de Medidas (ICRU) recomenda o uso de dosímetros com sensibilidades distintas para cada componente do campo. Os dosímetros termoluminescentes (TLDs) apresentam-se como uma alternativa para a realização da dosimetria de campos mistos de nêutrons e gamas em particular à utilização do par de TLDs de LiF TLD 100 e TLD 700, que apresentam respostas distintas às componentes de campo em virtude da diferença na quantidade do isótopo 6Li em suas composições. Porém, esta escolha apresenta algumas dificuldades pois a característica da resposta dos TLDs para cada componente de campo ainda não é totalmente compreendida. Este trabalho apresenta primeiramente um estudo de um sistema para moderação de uma fonte de AmBe para realizar os estudos de sensibilidade e linearidade dos TLDs quando irradiados em um campo misto de nêutrons e gamas. O sistema de moderação se faz necessário pois a fonte de AmBe emite preferencialmente nêutrons com alta energia, e a sensibilidade dos TLDs de LiF é preferencialmente para nêutrons de baixa energia. Entretanto, um dos objetivos do Grupo de Pesquisa em Física Médica do CEN/IPEN é a realização da dosimetria de campos mistos de alta intensidade, como por exemplo, o campo proveniente de um reator nuclear. Dessa forma esse trabalho realizou um estudo das respostas dos TLDs 100 e TLDs 700 quando irradiados no interior do núcleo do reator IPEN/MB-01 em duas diferentes configurações: cilíndrica com \"flux trap\" e retangular num arranjo de 26 x 28 varetas combustíveis. Esse trabalho contou com simulações com o código de Monte Carlo, MCNP5, para fornecer os fluxos e doses devido a cada componente de campo ao qual os TLDs estariam expostos. E a partir dos dados obtidos tanto através das simulações, quanto através dos experimentos, foi proposta uma metodologia para a utilização do TLD 100 para a dosimetria de nêutrons em campos com alta fluência de nêutrons, como é o caso do núcleo do reator IPEN/MB-01. / Mixed radiation field dosimetry is a research area that presents a great opportunity for studies due to the increased use on medical procedures such as proton therapy and Neutron Capture Therapy (NCT), as well as the importance of calculating occupational doses and radiation fields in nuclear reactors. For the dosimetry of mixed fields of neutrons and gammas, the International Commission on Radiation Units and Measurements (ICRU) recommends the use of dosimeters with distinct sensitivities for each component of the field. Thermoluminescent dosimeters (TLDs) are presented as an alternative for the dosimetry of neutron and gamma-mixed fields. In particular, the use of LiF TLD pair TLD 100 and TLD 700, which present distinct responses to the field components due to the difference in the amount of the 6Li isotope in its compositions. However, this choice presents some difficulties because the characteristic of the TLD response for each field component is not fully understood. This work presents a study of a system for moderation of an AmBe source to perform the screening and linearity studies of the TLDs when irradiated in a mixed field of neutrons and gammas. The moderation system is necessary since the AmBe source preferably emits neutrons with high energy, and the sensitivity of the LiF TLDs is preferably for low energy neutrons. However, one of the objectives of the Monte Carlo and Dosimetry for Medical Physics Research Group of CEN / IPEN is to perform the dosimetry of high intensity mixed fields, such as the field from a nuclear reactor. Thus, this work carried out a study of the responses of the TLDs 100 and TLDs 700 when irradiated inside the core of the IPEN/MB-01 reactor in two different configurations: cylindrical with flux trap and rectangular in an arrangement of 26 x 28 fuel rods. This work relied on simulations with the Monte Carlo code MCNP5 to provide the fluxes and doses due to each field component in which the TLDs would be exposed. From the data obtained, both through the simulations and through the experiments, a methodology was proposed for the use of the TLD 100 for the neutron dosimetry in fields with high neutron fluence, such as the IPEN/MB-01.